1. Field of the Invention
The present invention relates to a compact extended-base tension leg platform (the term tension leg platform is sometimes referred to as a xe2x80x9cTLPxe2x80x9d) substructure for supporting an offshore platform. The apparatus of the invention includes a plurality of support columns disposed about an open zone centered about a central axis of the substructure, a plurality of interconnecting pontoons, a plurality of stabilizing wings or arms for fixedly or removably securing a plurality of tendons anchored to the seabed, where columns are preferably symmetrically disposed about the central axis.
More particularly, the present invention relates to a compact extended-base tension leg substructure for supporting a platform which includes a plurality of support columns disposed about an open, wave transparent zone centered about a central axis of the substructure where adjacent columns are interconnected by at least one pontoon, where columns are preferably symmetrically disposed about the central axis. The substructure also includes a plurality of stabilizing wings or arms radiating outwardly from the columns and/or pontoons, where each wing is designed to fixedly or removably secure at least one tendon anchored to the seabed. Each column comprises an above water and submerged portion. The apparatus of the substructure minimizes or at least reduces translational movement and rotational flex in the substructure thereby reducing flex fatigue in the tendons anchoring the substructure to the seabed. The apparatus also de-couples tendon spacing and column spacing The present invention also relates to platforms incorporating the substructure, methods for making the substructure, methods for mooring an offshore platform, and methods for reducing the fatigue and extending the useful life of the anchoring tendons and connections.
2. Description of the Related Art
Many substructures have been described in the prior art. Many of these substructures are so-called large platform support structures that anchor to the seabed by means of an array of tendons. These tendons form a pattern that define the boundaries of a relatively large area of the seabed. Compact substructures are also known in the art, but they generally employ a central column with radially disposed arms. Such large and compact platforms are disclosed in the following U. S. Pat. Nos: 3,982,492, 4,421,436, 4,793,738, 4,913,233, 4,938,632, 4,983,073, 5,147,148, 5,381,865, 5,421,676, 5,431,511, 5,433,273, 5,549,164, 5,507,598, 5,567,086, 5,669,735, and 5,775,846, incorporated herein by reference. However, these structures do not include features of the present invention. For example, these structures do not include an array of arms or wings that radiate outwardly from a multi-columned, wave transparent substructure that minimizes or at least reduces the fatigue of the anchoring tendons. Thus, there is a need in the art for a multi-columned, compact, wave transparent substructure that minimizes or at least reduces tendon fatigue and that has an anchoring pattern on the seabed similar to a large tension leg platform substructure.
The present invention provides a compact, multi-columned, centrally wave transparent extended-base tension leg platform substructure for supporting an offshore platform. The apparatus of this invention includes a plurality of support column disposed about an open zone centered about a central axis of the substructure and at least one buoyant pontoon interconnecting adjacent columns where the columns are designed to engage and support a platform, where columns are preferably symmetrically disposed about the central axis. In operation each column has a submerged and a non-submerged portion and, along with buoyant pontoons, which are submerged, can be, and preferably are made selectively buoyant by means of ballast control. The substructure also includes at least one wing or arm fixedly attached to or integral with each column or each pontoon. Each wing or arm is attached to at least one tendon that is anchored to the seabed. The wings can be closed, opened or mixed structures (closed and opened parts), where the closed wings or wing parts can be separately ballasted.
The present invention also provides a compact TLP substructure for supporting an offshore platform which includes a plurality of support column forming an opened, wave transparent zone centered about a central axis of the substructure where adjacent columns are interconnected by buoyant pontoons, where columns are preferably symmetrically disposed about the central axis. The substructure also includes a plurality of wings or arms radiating out from the columns and/or pontoons, where each wing fixedly or removably secures at least two tendons anchored to the seabed, with each tendon engaging an opposite lateral side of a wing or arm. Each column includes an above water and submerged portion and, along with the buoyant pontoons, which are submerged, can be, and preferably are made selectively buoyant by means of ballast control. The substructure is designed to minimize translational movement and rotational flex in the substructure thereby reducing flex fatigue in the tendons anchoring the substructure to the seabed and to reduce flex fatigue in the connection members that attach the tendons to the wings and to decouple the tendon porch horizontal separation from the topside deck dimension. The substructure is also designed to provide a sufficient moon pool dimension to accommodate conventional top tensioned risers and direct vertical access to wells.
The present invention also provides a work platform and an equipment platform supported by the substructure of the present invention which includes platforms fixedly or removably attached to the substructure, previously described, the substructure, and the tendons anchored to the seabed. The platform can support drilling equipment, well completion equipment, risers extending from a well bore at the sea floor and upwardly through the open zone of the substructure to the platform, and other well-related equipment.
The present invention also provides a method for supporting and mooring an offshore platform to reduce fatigue in the anchoring tendons and their connections, the method including the steps of supporting an offshore platform on a substructure of the present invention, ballasting the substructure so that portions of the columns of the substructure are above the water and portions of the columns are below the water, and positioning a plurality of tendons so they are anchored at one end to the seabed and attached at another end to wings on the substructure.
The present invention further provides a method for making the substructures of the present invention including the steps of interconnecting adjacent support columns with at least one submergable pontoon, attaching at least one wing to each column or pontoon, attaching tendons at one end to the wing and at another end to a seabed anchor.